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analysis" and "reverse quantification," which were developed by the National Park Service (NPS), and others. These terms may be freshly minted, but the underlying conceptual framework is not new. In reviewing a claim for a federally reserved water right for all of the remaining natural flows of the streams in a national forest, the Idaho Supreme Court observed in 1978 that: "The periodic natural variations in stream flows with just the flexibility which nature provides without interference by man are said to fulfill the varying needs of the United States more effectively than could be done by any attempt to specify these varying needs in terms of cubic feet per second or acre feet" (Avondale Irrigation District v. North Idaho Properties, 577 P.2d 9, 19 (Idaho 1978)). HOW MUCH OF A RIVER'S FLOW SHOULD BE PROTECTED Instream flow protection begins with the question of how much water should be left in the stream to meet ecosystem needs and not developed to meet agricultural, industrial, and municipal demands. The early answers to this question were simple and minimal so as to leave as much water as possible for development. These answers were often based on the flow needs of a single, sometimes nonnative, fish species and were expressed as a single, year-round flow value, which was keyed to one life stage of that species or one component of its habitat. The flow needs of a river ecosystem are now increasingly recognized as variable and complex. Each river has its own flow history with patterns that vary from hour-to- hour, day-to-day, season-to-season, and year-to-year, in terms of magnitude, frequency, duration, timing, and rate of change of flows (Poff et al. 1997). The flows of one river may fluctuate greatly, influenced primarily by precipitation events, whereas another river may remain relatively steady due to a high groundwater table. Flow variables can significantly shape the ecology of a river, and flow variation in general has been called a "master variable" that "limits the distribution and abundance of riverine species, and regulates the ecological integrity of a flowing water system" (Poff et al. 1997). Native species may need a range of flow variations to complete their life cycles and may depend on seemingly harsh flow events such as floods and droughts. The complexity increases when one considers that not only must there be a mix of high and low flows, but that the duration of these events must vary in length. The natural ecosystem of any river is the product of millions of years of adaptation and evolution, which have created a myriad of variables and subtleties more complex than we can imagine. With each additional habitat type or species to be protected, determining how much water should be left in the stream to meet ecosystem needs is compounded and confounded. It soon becomes evident that one way to answer the question is to systematically characterize the natural flow patterns for a particular river or stream and to apply all aspects of that pattern to the question (Richter et al. 1997). This answer to the question of how much water should be left in a stream and not developed is known as the natural flow paradigm. This answer follows more readily if the question of how much water should be protected in the stream is turned upside-down and asked in terms how much can we depart from the natural patterns of a particular 2